Heart Rate Variability (HRV). HRV has two domains, time domain and frequency domain. SDNN (Standard Deviation Normal to Normal) is the first time domain. In healthy state, the signal is more diversity and SDNN is high. Decrease of SDNN means that ability to maintain homeostasis is reduced. It is shown while a person is in any disease state. Significant decreasing of HRV is an indicator for general health. PSI (Physical Stress Index) is the pressure loaded state due to overloaded physical activity and heart rate is increasing. Total power (TP), Very low frequency (VLF), Low frequency (LF), High frequency (HF), normalized Low frequency (nLF, LF norm) = LF/(LF +HF), normalized High frequency (nHF, HF norm) = HF/(LF+HF) and Low frequency to High frequency ratio (LF/HF) are frequency domain. Mean Heart rate (MHR) is also an indicator representing autonomic function.